Fluxes comprised of dicarboxylic acids as fluxing agents are described in the seminal U.S. patent of Ernest G. Tiegel, U.S. Pat. No. 3,424,625. Solutions comprising a major portion of dicarboxylic acids dissolved in a minor portion of organic solvents such as lower alcohols, ethers, aldehydes, ketones and dimethyl sulfoxide are described in the Tiegel patent.
U.S. Pat. No. 4,708,751 discloses low levels of dicarboxylic acids (less than 3.5%) dissolved in organic solvents and other ingredients including low levels of rosin (less than 2%).
Halogen-free fluxes which are also rosin-free utilizing organic acid based fluxing agents are also described in the art.
U.S. Pat. No. 4,601,763 describes rosin-free and halogen-free fluxes containing low levels of dicarboxylic acids (3%) dissolved in organic solvents(s).
In U.S. Pat. No. 5,085,365 the environmental problem with using chlorofluorocarbon (CFC) solvents to remove rosin flux residue is addressed. CFC solvents evaporate into the atmosphere and cause the depletion of the protective ozone layer in the stratosphere. U.S. Pat. No. 5,085,365 describes the use of polycarboxylic acids such as citric acid as fluxing agents. Where the fluxes of U.S. Pat. No. 5,085,365 are used, rinsing is required--and thereafter, the rinse water needs to be processed because it is necessary to remove acid flux residue therefrom.
Another alternative to using rosin fluxes and cleaning with environmentally detrimental solvents is to use fluxes with a low solids content (less than 6%) as described in U.S. Pat. No. 4,708,751, where low levels of fluxing agent, namely, less than about 3.5% are described. The flux described in U.S. Pat. No. 4,708,751 is known in the electronics industry as a no-clean, low-residue or low-solids flux. No rinsing is required for this type of flux.
The current and developing problem with low-residue fluxes of all types is that such fluxes are based on alcohol and/or other volatile organic compound solvents. Such fluxes are applied to the bottom of a printed circuit assembly by pumping the flux into a wave, spraying or passing air into the flux to create a foam head and then conveying the printed circuit assembly over the flux. The next step in the process is to convey the assembly over or through a preheater to evaporate away the volatile solvent and activate the flux. Then the assembly is conveyed across a melted solder wave or pot. This rapid evaporation of the volatile solvent permits high speed conveying and soldering without spattering when the assembly touches the melted solder.
Volatile Organic Compounds (VOCs) have been described in the Federal Register (Vol.55, No.126 Friday, Jun. 29, 1990) Rules and Regulations as "any organic compound which participates in atmospheric photo-chemical reactions.". Already the State of California has recognized that VOCs emitted into the lower atmosphere contribute to the formation of smog by photochemical reaction and requires record keeping of VOC emissions.
Fluxes in use by the electronics industry for mechanized soldering of printed circuit assemblies either contain undesirable VOCs or, if water-based, contain conductive and/or undesirable levels of corrosive organic acids which must be removed with water after fluxing.
The present invention provides a VOC-free, no-clean, low-residue flux using a demineralized water-based solvent system and a foaming system which includes a nonionic surfactant and a nonvolatile organic component that is a cosolvent with the demineralized water.